add option or setting to enable writing time-stamps for beginning of period

[andywood]

Example: when writing daily mean outputs, the time stamp is for the end of the day, so it registers as the next day when used by applications at a daily level (eg mizuRoute) and must be shifted in a post-processing step to denote the date of the data. e.g., a run starting 10/1 00h will write its first daily output on 10/2 (0h).

Proposed change: probably add to the output_control file, similar to 'double/float' precision setting. Make backward compatible (ie if missing, assume current behavior). Would not affect instantaneous outputs.